Engineering plastics have found wide applications in automobile, precision instruments, electronics, electrics and communications. However, the requirements of superior performance in heat & chemical resistances, flame retardance, electrical, optical & mechanical properties . . . are getting severe. In the past, the study of high performance engineering plastics was centered on benzene ring structure, the newly developed high performance polyalkylene naphthalate (PAN) with their superior physical, mechanical properties and processability have caught the world's attention. The most important feature of PAN polymer is the increased stiffness of the polymer chains due to the presence of naphthalene ring instead of the benzene ring in polyalkylene terephthalate (PAT).
Of polyalkylene naphthalates, polyethylene 2,6-naphthalate (PEN) and polybutylene 2,6-naphthalate (PBN) have superior thermal, mechanical, barrier and chemical resistance properties relative to, for example, polyethylene 1,4-terephthalate (PET) and polybutylene 1,4-terephthalate (PBT). For this reason, numerous research works are currently underway to develop commercial applications for these new high performance polyesters. PEN has found many applications: Higher tensile strength and dimensional stability makes PEN film an excellent choice for the manufacture of magnetic recording tape and electronic components. Additionally, because of its superior resistance to gas diffusion, and particularly to the diffusion of carbon dioxide, oxygen and water vapor, films made from PEN are useful for manufacturing food containers, especially the so-called "hot fill" food containers. PEN can also be used to prepare high strength fibers useful for the manufacture of tire cord. With regard to PBN, because of its crystallization rate is much faster than that of PBT (1 second vs. 3 seconds), therefore, its processing cycle time can be shortened. Because of its facile processability, PBN can be molded either by injection or extrusion into electrical, electronic and machine parts. PBN with excellent mechanical strength, heat resistance, dimensional stability, resistance to chemical, acid and alkaline and impermeability to gasoline and gasohol is particularly suited for making into automobile parts that come in contact with fuel, oil, combustion gas. In addition, fiber reinforced PBN for high temperature and humidity circumstance has been reported. However, both PEN and PBN have a very poor solubility in most of the common organic solvents, for example they are insoluble in N-methyl-2-pyrolidone (NMP), dimethyl sulfoxide (DMSO), m-cresol, pyridine, and N,N-dimethylformamide (DMF), etc., which renders PEN and PBN ineligible for certain applications.
The main object of the present invention is to provide copolyesters containing a naphthalene ring structure which not only have the superior mechanical and thermal properties exhibited by the naphthalene ring structure, but have improved solubility due to an additional aryl ether linkage being incorporated to the main chain thereof. The improved solubility facilitates the workability of the present copolyesters in various applications such as paint, varnish and structural adhesive.